There is a need for efficient storage of thermal energy within the area of modern energy technology.
Thermal energy may advantageously be stored in a fluid, such as e.g. water, above ground in insulated tanks, in ground in insulated pits, or underground in excavated caverns, using the surrounding ground as insulation. The thermal energy of the fluid is preserved to a great extent during an extended period of time. Today, these methods are used in different parts of the world in order to satisfy the need for storing thermal energy between different seasons, e.g. storing temporary surplus heat which is used later on when there is a demand for it and, preferably, when its financial value is higher. The main transition of energy is from the summer half, when there is less need for heating, to the winter half, when the need for heating is much higher. However, there is also much to gain by using the storage for short-term variations and always actively storing surplus heat. These kinds of storages may also be used for storage of a colder fluid, to be used for cooling, as well as for fluid having an intermediate temperature, such as a fluid used in low temperature systems.
A substantial disadvantage with the arrangements for storing thermal energy underground which are available on the market today is that they, during use, contain large amounts of energy having an intermediate temperature. The intermediate temperature is not high enough to be used for different types of heating and it is not low enough to be used for cooling.
Swedish patent application 0950576-9 discloses one kind of efficient storage of thermal energy. However, there is still a need for an even more improved arrangement for storing thermal energy underground.